Response of ground-penetrating radar to bounding surfaces and lithofacies variations in sand barrier sequences

Abstract

This paper investigates the amplitude response of ground-penetrating radar to typical lithofacies bounding surfaces in a sand barrier sequence. Ground-penetrating radar reflection coefficients of typical bounding surfaces in a sand barrier sequence are computed using time-average and Hanai-Bruggeman-Sen effective-medium mixing laws for the dielectric properties. The effects of lithological contrasts due to packing, porosity, grain shape and orientation, mineralogy and fluid content are calculated. Reflection coefficients are typically 0.05 to 0.1 but may range up to 0.45 for large dielectric permittivity contrasts. For a radar example recorded over a beach strandline, the relative reflection amplitude alone is generally insufficient to uniquely determine a specific type of bounding surface between two lithofacies. Other attributes of the section, such as apparent dip magnitude and direction and the external form, help to refine the interpretation. This interpretation approach is formalised by defining radar facies units as 'groups of radar reflections whose parameters (configuration, amplitude, continuity, frequency, interval velocity, attenuation, dispersion) differ from adjacent groups'. Radar facies are distinguished by types of reflection boundaries, configuration of the reflection pattern within the unit, and the external form or shape of the unit.